How High-Frequency Welding Works
In high-frequency welding, steel strip is slit, uncoiled, leveled, butt-welded, and formed into a cylindrical tube blank. The blank then passes through an induction coil or contact welding point, where current induced in the strip edges heats them by resistance. The heated edges are pressed together by squeeze rolls to form the weld. No filler metal or welding wire is added, so the process is essentially forge welding, and the squeeze amount is the most important control variable. If the squeeze is correct and stable, no molten metal or oxide remains on the fusion surface. The heat-affected zone has a characteristic waist-drum shape, and the metal streamline rise angle visible on metallographic samples is used to evaluate the degree of upsetting.
The Nine Common Defects
1. Inclusions (black overburned oxide). Metal oxides form at the V-shaped opening and are trapped on the fusion surface instead of being expelled with the molten metal. The fracture is flat with no metallic luster and may appear singly or in a chain. Risk increases when the V-opening angle is below 4 degrees or the Mn/Si ratio of the strip is below 8 to 1. Prevention: hold the V-opening angle between 4 and 6 degrees, keep tooling and equipment aligned, use a relatively low welding temperature, and avoid Mn/Si ratios below 8 to 1.
2. Pre-arc (white overburned oxide). Burrs or rust falling in front of the V-opening apex form a bridge that short-circuits the current, causing an arc jump that reduces heating at the joint. The defect is small, usually no longer than the wall thickness, and appears as a bright flat plane surrounded by fibrous fracture. Prevention: control the V-opening angle between 4 and 6 degrees, reduce edge trimming burrs, protect the strip edges from damage, and keep cooling water clean and away from the V-opening.
3. Insufficient fusion (open seam). The strip edges are heated but not completely fused, leaving a visible slit; the open seam edge is bluish, indicating heating without bonding. The direct cause is insufficient weld heat input, but squeeze amount, V-opening geometry, impeder position, and cooling conditions also contribute. Prevention: match heat input to material and speed, position the impeder beyond the squeeze roll center by about 3.18 mm, keep the V-opening length no longer than the pipe diameter, keep the V-opening angle within 7 degrees, limit the clearance between induction coil and pipe to 6.35 mm, and use strip width suited to the pipe diameter.
4. Insufficient edge fusion (edge waves). Fusion is incomplete at the outer or inner edge of the seam, often caused by the weld being crushed and cracked or by bulging that leaves the outer edge colder than the inner. The fracture is flat without metallic luster, or silver-gray in the bulging form. Prevention: keep the strip edges straight and parallel, use a larger squeeze amount, and increase heat input when the bulge fracture is silver-gray.
5. Mid-wall insufficient fusion (cold welding in the middle). The middle of the wall thickness shows a flat silver-gray band with fibrous edges after failure, caused by the welding power being insufficient for the line speed or by incomplete discharge of molten metal. Prevention: increase welding machine power, increase squeeze, lengthen the V-opening, or reduce welding speed.
6. Sticky welding (cold welding). The joint surfaces are pressed together without true fusion. This defect is the most dangerous because it transmits ultrasonic signals and cannot be detected by electromagnetic testing; it cracks under flattening with a flat brittle fracture. Metallographic examination shows a very narrow heat-affected zone, no white fusion line, and a small streamline rise angle. Prevention: use sufficient welding power for the material and speed, ensure full squeeze, and increase strip width.
7. Cast welding (brittle welding). Molten metal remains on the fusion surface instead of being expelled, leaving cast metal containing oxides; the fracture is flat and brittle and cracks when flattened. Prevention: increase welding discharge and strip width.
8. Porosity (pinholes). Pores on the joint surface are caused by high welding temperature with insufficient discharge; the fracture is fibrous with bright white spots distributed across it. Prevention: reduce welding heat input and increase squeeze.
9. Jump welding. Regularly spaced defects at intervals equal to a multiple of the 60 Hz power frequency, appearing as wavy discontinuities on the outer wall. Prevention: improve filtering of the welding current, check the input phase voltage, and inspect rolls and shafts.
Root-Cause Analysis in Practice
Defects in actual production are usually the combined result of several factors. A narrow V-opening does not necessarily produce burnt oxide unless the squeeze is below the normal requirement, and a small squeeze can come from slightly narrow slitting width, worn tooling, or improper equipment installation. Some causes sit outside the weld area: for example, cavitation in the cooling pump can leave the impeder under-cooled, the impeder heats instantly, the current concentrates less at the V-opening, and cold welding results. The most effective practice is to identify root causes systematically and record operating parameters such as strip working width, welding speed, coolant flow and pressure, grid flow, and squeeze amount, then monitor the records for abnormal fluctuations.
Preventive Quality Control
Most welding defects can be traced to improper installation or adjustment of the forming and welding unit. A reliable manufacturing plan, daily monitoring of operation records, regular training of high-frequency welders, improved trimming and edge processing, proper coil storage, and preventive maintenance of tooling all reduce defect rates. Together these measures keep the seam quality consistent and minimize scrap and rework on the high-frequency welded pipe line.
Frequently Asked Questions
Which high-frequency welding defect is the most dangerous? Sticky welding, or cold welding, because the joint transmits ultrasonic signals and passes electromagnetic testing while remaining unfused, so it only reveals itself under flattening or in service.
What does the metal streamline rise angle indicate? It reflects the degree of upsetting during welding; a very small rise angle is a sign of insufficient squeeze and possible cold welding.
Why does a narrow V-opening angle increase inclusion risk? When the angle is below about 4 degrees, molten metal and oxides cannot be expelled completely through the joint, so they are trapped on the fusion surface as inclusions.
Can jump welding be eliminated by machine maintenance? Mostly yes. Jump welding is linked to current filtering and mechanical condition, so upgrading current filtering equipment, checking input phase voltage, and inspecting rolls and shafts address the common causes.
Why is the heat-affected zone darker than the base metal? Carbon diffuses toward the strip edges during welding; carbon near the edge is oxidized to CO or CO2, and the remaining decarburized iron appears lighter, while the carbon-enriched zone appears darker in the waist-drum shape around the seam.
